The present invention relates generally to oscillator circuits, and more particularly, relates to a ring oscillator and methods for evaluating dynamic circuits.
Measuring circuit delay is an important metric of a design or technology. A common method to measure delay is connecting a series of identical circuits together into a ring so that the overall path is inverting.
Ring oscillators typically include a series of devices or stages connected together to form a ring with a feedback path provided from the output of the last of the series of devices to an input of the first of the series of devices. The devices may include logic gates, inverters, differential buffers, or differential amplifiers, for example. Any inverting path with sufficient gain will oscillate when connected in a ring, while a non-inverting path will simply lock on a particular starting logic level. The ring oscillator is essentially a series of stages, each stage having an intrinsic delay from input to output. The frequency of the ring oscillator output is a function of the total delay time of the series of stages. The steady state frequency of the oscillator is dependent on the intrinsic delay of each of the stages.
FIG. 1 illustrates this conventional ring oscillator arrangement for measuring circuit delay. This prior art ring oscillator of FIG. 1 includes a series of multiple inverting stages connected in a loop, as shown. When power is supplied, this chain will oscillate at a certain frequency that indicates the sum of the delay through the entire chain. Dividing this delay by the total number of stages provides the delay through each stage. This is a common technique to evaluate static CMOS types of circuits. For a given stage, both the delay of the input rising and the delay of the input falling contributes to the oscillation frequency. This is usually the desired result for static circuits, since designers are interested in both phases of transition since each is equally important.
However, there are classes of circuits in which the measured performance of only one of the input transitions is desired. These classes of circuits include, for example, dynamic circuits, memory access paths, and the like.
A need exists for an effective mechanism for measuring independently one of the input transitions in dynamic circuits, memory access paths, and the like. It is desirable to provide an improved method and ring oscillator for evaluating dynamic circuits.
A principal object of the present invention is to provide an improved method and ring oscillator for evaluating dynamic circuits. Other important objects of the present invention are to provide such an improved method and ring oscillator for evaluating dynamic circuits substantially without negative effect and that overcome many of the disadvantages of prior art arrangements.
In brief, measurement methods and a ring oscillator circuit are provided for evaluating dynamic circuits. The ring oscillator circuit includes a one-shot pulse generator receiving a single transition input signal and producing a pulse output signal having a rising transition and falling transition. The dynamic circuit to be evaluated is coupled to an output of the one-shot pulse generator receiving the pulse output signal of the one-shot pulse generator and producing a delayed output pulse at an output. A divide-by-two circuit is coupled to the output of the dynamic circuit to be evaluated. An output signal of the divide-by-two circuit is fed back to the one-shot pulse generator, and the cycle is repeated, thus oscillating.
In accordance with features of the invention, a multiplexer is connected between output of the dynamic circuit to be evaluated and the divide-by-two circuit. The multiplexer receives the pulse output of the one-shot pulse generator and includes a select input for selecting the output of the dynamic circuit to be evaluated or the pulse output of the one-shot pulse generator. By inserting the evaluation circuit into a path that can be multiplexed in and out of the oscillator path, and by measuring the difference between the frequency with and without the evaluation circuit in the path, the performance of the evaluation circuit can be accurately determined.